


Mood Lighting

by skerb



Category: Undertale (Video Game)
Genre: Desk Sex, Don't copy to another site, Don't get caught, Ecto-Vagina (Undertale), Established Relationship, Fluff and Smut, Fondling, M/M, Making Out, Multiple Orgasms, Office Sex, Porn with Feelings, Sanster, Scientist W. D. Gaster, Sensitive bones, Soul Sex, Tentacle Sex, Undertail, Vaginal Fingering, W. D. Gaster is not related to Skelebros, but i guess it's on the down low
Language: English
Status: Completed
Published: 2019-01-06
Updated: 2019-01-06
Packaged: 2019-10-05 07:14:01
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 6,343
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/17320409
Author URL: https://archiveofourown.org/users/skerb/pseuds/skerb
Summary: A power outage at the lab results in the perfect opportunity to get close after a long night, and Sans and Gaster make the most of it, even under threat of surveillance.





	Mood Lighting

**Author's Note:**

  * For [raritysdiamonds](https://archiveofourown.org/users/raritysdiamonds/gifts).



Gaster had been silent. It wasn’t anything unusual. His day started with an acknowledging nod to his coworkers early in the morning when they got in, carrying a mug of coffee as he made his way back into the inner cloisters of the lab. Under his opposite arm was a pile of reports on a clipboard, while another hand held a second mug of coffee. One more grasped a container of assorted snacks, carefully selected from the cafeteria.

Inside of his head was a plethora of reminders and troubleshooting in order to secure new ideas and explanations that he had to write up for the King, presented alongside his assistant.

Speaking of… if he knew the little devil well enough, he would be right where he’d left him.

The walk to the elevator leading to the lower walkway of the lab was much the same. The hallways echoed with each measured footstep and the hum of the elevator box was forever constant. It took a lot of guts for any monster to be surrounded by so much steel and magic all at once, but Gaster had made peace with it. The central hub of monsterkind’s most powerful technology was the foremost in safety protocols, after all. Why should he pay attention to other monsters’ worries when he felt so much at home?

 

Despite the cords still hanging on the walls being so new and haphazardly thrown to the side, Gaster stepped over them when he exited the elevator shaft and made his way down the brightly lit corridor. The absence of its usual keyboard tapping or nonsensical noises of rumination signalled the fact that perhaps Sans had either gone home, or he had fallen asleep. Ultimately, Sans had stayed behind, citing that he still had reports to work on.

Gaster conjured another hand as he regarded the door in front of him. It was more different than usual - while its smooth, wooden structure was entirely the same as the previous evening, right down to the brass knob, something had… changed. It hummed with electric life when he approached to open the door and he tilted his head. He stared at it when the door gave a soft hiss, the tile under his foot depressing with a quiet  _click,_ and seconds later the wood panel slid to the side into the wall.

Well, that certainly was new. As Hotland’s puzzles changed daily, there was no need for alarm. Not one to be normally worried by such silly little discrepancies in lab mechanics, Gaster stepped inside. Once in the room, the door hissed once more as it closed smoothly behind him.

Interesting.

His eyes settled on the still form of his colleague, best friend and for awhile now… something else entirely. He had figured as much, and despite Sans being so hopelessly predictable, it still warmed Gaster’s soul that the skeleton fell asleep so soundly.

With a myriad of detached appendages at his disposal, the container was set on the desk where Sans’ arms were folded and his skull was resting in. The keyboard was pushed askew and Gaster had to grimace at the 654 pages of “bbbbbbbbbbbbb” from Sans’ fingers resting on the keys in his sleep. Gaster passed his mug and clipboard to a pair of standby hands and drew nearer to the sleeping skeleton, enough to detect the subtle shift of his rib cage rising and falling with every breath. It was simply calming to hear the subtle whistle leaving his nasal aperture.

With how deeply the skeleton slumbered, Gaster had come to realise earlier on in their relationship just how impossible it was to wake Sans under normal circumstances. So there he lay and even curled against his body when Gaster put his arms around him and scooped him up. A soft sigh, a nuzzle, and the most esteemed genius behind the grandest ideas had to physically stop himself from grabbing at his chest in mock  _pain,_ he was so adorable.

Of course, Sans would never admit it. But Gaster thought the man part koala, the similarities were uncanny. Especially with the way the skeleton would automatically latch onto him when Gaster held him in his arms and moved to sit down. Peaceful and carefree. Gaster really didn’t know how he came to be so lucky.

Gaster leaned back in the chair as he allowed Sans’ bones to settle against him, feeling how his body gave just a little in order to cradle Sans as he slept. He gently traced and followed the curvature of the skeleton’s spine. It was so different than his own, and although he saw it with broadened scientific curiosity, Gaster still felt at odds with himself for even permitting himself this amount of exploration when Sans wasn’t awake.

Sans’ overcoat gave just a little bit more as Gaster’s deft fingertips traced the spines in his vertebrae. He noted how the skeleton’s gasps stuttered with the disturbance and he couldn’t help the devious smile that crept onto his face. Gaster knew exactly what was happening and he planned to not give in to the little skeleton’s game so soon - even if it was during working hours. The two of them normally stayed well past what was considered acceptable shift hours, so a brief respite was permitted here and there.

He laid his palms flat against the skeleton’s back, feeling the warm porcelain underneath and the subtle, repressed laugh that shook Sans despite how he still wanted to appear asleep. Gaster decided to test his limits, idly stroking with both palms until Sans’ body relaxed further, melting against him contentedly with a soft sigh. Then he stilled into natural sleep once more.

There was that mental chest-clutching again, Gaster thought.

Evidently, he had lost that game - and that game was forever pranking each other in one way or another. It was just that this morning it seemed a little more tender, and dare Gaster even think it -  _affectionate._

“You may want to wake up, Sans,” Gaster whispered against Sans’ skull. He was only met with a brief snore in retort, so he continued outlining the skeleton’s spine, surveying every subtle jump and twitch. “How late did you stay up?”

“i dunno. what time’s it now?” Sans grumbled from somewhere between Gaster’s lab coat and sweater.

“Dare I even respond to that, you’ll just lie for my benefit.”

“ok. between midnight and now, then.”

Gaster clicked his tongue softly as though in chastisement, but didn’t stop from his digits’ explorations until he felt Sans suddenly jolt the lower he got. Flushed, Sans lifted his head, grabbing the conjured hand behind him while still looking very much half-asleep.

“jeez, you’re really handsy today,” the skeleton muttered as he uprighted himself in Gaster’s lap. He took a precursory glance around him at one, two… six of them, all at the ready and greeting him. Their fingers wriggled teasingly and Sans had to laugh as he rubbed at his eye socket.

“And that differs any other day, how?”

Sans smirked, it devolving into a loud and audible yawn. Gaster took that as a silent plea for coffee and diligently held one of the mugs for Sans to take. It took a good nudge or two to get the skeleton’s motor skills going again, but in the end his assistant was well on his way to demonstrating he still knew how to utilise his opposable thumbs, even with his eye sockets slowly drooping.

He watched as the skeleton drew the steaming hot mug to his teeth, his magic taking on a subtle hue as he drew from the vessel and it was absorbed into his body. As with anything he had come to reflect upon before, Gaster reflected on the seemingly mundane, everyday things Sans did with adoration.

My, he really  _was_ smitten.

Sans cocked a grin at him, a little more rejuvenated after downing a good portion of the coffee. As though it hadn’t been enough, he closed his eye sockets and raised the mug again, only to choke in surprise when Gaster settled his cored palms over Sans’ hip bones with a squeeze.

“that’s sure a way to wake up in the morning,” Sans observed as he wiped his jaw with the back of his carpals.

“Just some casual motivation. Should we garner out the flex tube repositories today, perhaps we could endeavour in more… lax activities.”

“fancy way of sayin’ `hey sans, let’s be lazy tod-`” Sans would have said more if not for the fact the well-lit and normally bright room suddenly flickered, went pitch dark and a low whine of electricity petered off into nothingness. They were silent and still for a moment, daring not to move until the CORE stabilised and went back online. Sometimes it was a fluke and it needed a cooldown period, other times it just needed to restart.

Gaster hoped it was either of those reasons and not that something horrible had happened to his dear project.

“..ay,” the skeleton finished after a precursory look around. While there were backup generators, the waning thrum of electricity shutting down all around them still rung in their heads. Gaster heard the subtle shift of bones clattering together as Sans heaved a sigh and clambered off his lap.

The royal scientist could see the vague outline of Sans’ body in view as he quietly rose to pursue his assistant.

“i’m on it, i’m on it…” The smaller monster barely suppressed a yawn and fumbled around the lab for what was needed.

Gaster never quite understood how the skeleton could shamble around in the dark without as much as a pinprick of light to see where he was going, but he found it interesting nonetheless. It was very likely that Sans had spent much of his life in a half-awake state that the voids where his eye lights shone kept the darkness under somewhat of a familiarity. Or perhaps there was a reason why the skeleton could still utilise sight even when said eye lights dissipated under scrutiny, distress, or any wide and expressive variance of emotion.

Very interesting. Perhaps he would enquire as to the teratological metamorphosis his species had undergone throughout their time trapped in the Underground…

As he pondered the logistics, Sans rummaged around in the dark. Gaster heard the faint clicks of glass on bone alongside the nonsensical, whimsical humming coming from his lab assistant.

Then, very abruptly, he heard the unmistakable  _thunk_ of bone on steel and a muffled curse. Shortly thereafter, a flickering string of faerie lights was revealed no more than a dozen or so feet away.

Gaster gave him a perplexed look, drinking in the bright dots of light that littered Sans’ body. The twinkling minuscule pips of light were intermittent, just bright enough to illuminate the mischievous grin that widened against the smaller monster’s bony features. It resulted in a vague tug at the royal scientist’s mouth, but he resisted with all of his might.

“What is this?”

Sans grinned back at him, as though he’d been excited to deliver a joke since he had first thought of it. “mood lighting.” He was rewarded with a scoff, but it wasn’t berating.

“However did you bring those in without my detecting them?” Several of his hands lifted in the air, all making different enquiring gestures as Sans rocked on one foot. He observed the skeleton’s smug nature, how his eye lights casted a soft glow in his sockets and how the string of lights revealed his fidgeting. After a moment’s pause, Gaster sighed quietly. “No matter. I suppose safety protocols dictate we make our way back up to the assembly rooms.”

Sans scratched at his jaw, then meandered back over to the desk instead. When he turned, the muted light revealed that his grin had broadened quite a bit.

“actually, about that…” he hesitated, then sighed. “that door… runs on power, dings.”

Gaster frowned, not understanding the grin. Was it perhaps out of nervousness, or that he had deduced that they were both trapped in the room until the power went on? Not even the generators would have enough power to supply, as they were used to filter magic into the laboratory in order to keep monsters inside stable during emergencies.

He groaned lowly, covering his face with one hand while a spare few others made disparaging gestures.

“hey, don’t worry bout it too much, doc,” Sans assured him, his voice oddly calm. “at least the cameras are offline, eh?” He laughed, his tone good-natured.

The doctor regarded one of the corners of the room, the high ceiling shrouded in darkness. The little red light indicating that they were under constant surveillance - for security purposes, he was told - was out. His frown deepened as he regarded the skeleton, now leaning back against the desk, decorated like a heap of Gyftmas decorations.

He decided to draw nearer so he could clearly see what was going on. Sans had wrapped a strand of lights around his shoulders and in between his collarbones, and once loosely around the vertebrae of his neck like a necklace. His lab coat was partially undone as it always was, but as Gaster had decided to look on, Sans gave him a cheeky grin and unfastened the next button.

How bold.

The collar slipped from his shoulder, just barely hanging on as he reclined. One of his eye sockets closed in a wink, as though he was attempting to look entirely sultry.

He wasn’t concerned about the power outage and the little skeleton had blatantly pointed out the lack of surveillance. Gaster buried his face against his prominent hand, as though doing so would effectively hide how flustered he felt at the unspoken invitation.

Gaster almost flinched when he heard the tapping from the desk, a sign indicating Sans was strumming his bony digits against the hardwood surface provocatively.

“doctor gaster, sir,” Sans murmured, mirth in his voice as he feigned utmost professionalism. He took a small screwdriver from the penholder near his hip and twirled it between his digits. “i’m afraid our calibrations aren’t going as planned… with an unsteady output of resources, you’re going to have to… get your hands dirty.”

If the skeleton had eyebrows, they would have been waggling comically by now. Gaster sighed into his cored palm, trying to cover up the fact that he was very close to breaking composure and cracking up. He managed to regain himself and very softly cleared his throat, manoeuvring around the large desk with a suspicious glance to the camera hanging in the corner.

“Oh my. Well, that simply will not do,” he replied in a hushed tone, taking the invitation for what it was.

There was only so much they could do whilst trapped in the lower labs, and escaping the room was not an option. Well, not an option insofar as the dim lighting was not optimal for the precise methods needed to unbolt the door from the wall and let them out. He watched, his soul fluttering when Sans’ phalanges tugged another button free, exposing one of his ribs. Gaster’s breath caught in his throat as he realised what had taken Sans so long to find the lights.

Not wanting to give the impish little skeleton the advantage, he considered it for a moment, watching as Sans toyed with the next fastening in idle thought.

“I know just the implements for such delicate equipment,” he found himself saying, his mouth quirking into a wider smile. Looking a little smug, Sans leaned back ever so slightly, daring him to continue. Gaster indulged the skeleton, one of his conjured hands teasing the fold of fabric hiding the ribs just out of view as another slipped under the lab coat’s tails.

He saw the way Sans hunched inward with a subdued shudder, restraining his surprise even at his invitation. Gaster straightened his back, surveying the monster before him while his hands went to work.

 

Though it certainly hadn’t been the first time he’d played with Sans in such a way, it had usually been after work hours, in stolen moments together. Yes, he was quite aware of the implications behind being caught on the job… but so rarely did they have the  _time_ to fool around. It was either that the two of them were exhausted by the end of the week, or they just never acknowledged that they needed to do other things besides research.

A broken mewl and the sound of the screwdriver falling to the desk and rolling off the side to the floor interrupted Gaster’s thoughts, a blush tinting his cheeks as he moved to get closer.

Alright.

If the lab was going to crash offline, he’d spend the time waiting with the one he loved and adored.

 

Sans’ weight rolled to his heels and he nearly collapsed onto the desk, very obvious that he was becoming quickly aroused. He huffed and grabbed at his front, pulling the fabric while his bony digits caught at the string of faerie lights. The warmth emanating from his soul made Gaster’s own kickstart as the ghostly fingers within Sans’ lab coat enclosed upon something very, very interesting.

And soft. Viscous - a little hot to the touch with enough yield to simulate a plush firmness that Gaster just needed to investigate. Sans covered his teeth with his free hand, his back slipping against the edge of the desk as the doctor closed in, ready to support the skeleton at a moment’s notice.

“don’t tell me,” Sans gasped, moving to grab Gaster’s shoulder to steady himself. His mouth hung open with a series of sharp, excited breaths. Its heat brushed past the side of Gaster’s hand as he caressed down Sans’ skull, tracing down every line and pit while the one hidden inside Sans’ rib cage explored further.

Sans’ toes curled as he muted a low moan of surprise and tilted his skull back. Gaster had taken to outlining the soft organ with slow, aching circles, making Sans squirm on the spot as the massaging pressure filled his bones with need. He clung to the doctor’s shoulder, his grip slipping as he was toyed with, soft sounds breaking the silence.

Gaster knew that Sans’ restraint was due to their times together being quick, hushed and hastily desperate between reports. Being recently put under surveillance made passing moments difficult to pass up, for any length of time.

“dings,  _ah-”_  Sans jerked as a third hand joined the teasing inside of his rib cage, pulling needy and eager pants from him. His voice stuttered on a restrained word, just cusping on the edge of demand. His tone slipped higher as he moved further into the throbbing ecstasy encompassing him, unable to keep himself from uttering any lewd and muffled sounds.  _“closer-!”_

 

Gaster’s body felt warm with the plea and he moved in, wanting to join in once more in perfect, yet cacophonous harmony. The sounds that Sans was rewarding him with were splendid and dare he have the heart for it, he’d give him a little tease for being so lewd at the office. Yet that was neither here nor there, and the doctor just couldn’t help the echoed pleasure he felt when Sans explored his chest, pulling at his clothes, his fingers then moving down and hooking into the belt loops of his trousers.

He hissed in response to the urgent tug, their hips connecting with a swirling heat as Sans wrapped his legs around Gaster’s waist while a skeletal hand jerked him down by his tie.

Gaster obliged him with a startled grunt, fumbling now under pressure. While he acted aloof and professional in all sense of the word on the clock, when it came to more personal endeavours, the doctor was rigid, clumsy, and flustered. It had taken a lot of coaxing for Sans to pursue a more intimate relationship, and utilising his collection of hands had led to discoveries previously unknown about how much a monster could take before becoming a useless, slobbering mess.

Which Sans was dangerously close to being now. His face was flushed as his soul was massaged, two of Gaster’s fingertips teasing the cleft of the organ as though it were softer features, splaying his soul as it wept into his palm and trickled slick down his wrist.

“My, already so wound up,” the doctor observed, his soul hammering as the skeleton pulled him down with another hearty tug.

 _“h.. hah,_ how are you n.. not affected by this,” Sans crooned, a shudder of pleasure making his body arch. He huffed when a fourth hand pulled down one side of his trousers while the fifth explored the join between his ischiums, coaxing his aching magic to form.

“Prefer to observe,” Gaster admitted truthfully. “To take in full view, to experiment-”

_“-ah!!”_

_“Tsk,_ you’re being quite rude, Sans. You should know better than to interrupt. How many demerits is that?”

The skeleton shuddered again, this time pathetically holding back another whine of protest. The lights made clicking noises as they brushed against his ribs and collar bones, flickering as they moved. Gaster still couldn’t quite figure what his reasoning behind it was, but it looked pretty all the same.

“none,  _mmfh-”_  Sans had pushed his face so close to Gaster’s that he could almost taste the magic dripping from his tongue.

Mouth quirking with indulgent self-satisfaction, Gaster stole a kiss, drawing upon Sans’ magic before very quickly pulling away again. Sans’ breathy moans were cut off short, and the doctor knew full well that he was still trying to restrain himself.

“y’got that the screwdriver was innuendo, right?” the skeleton murmured hastily, his eye sockets clenching when fingers traced a sensitive curve inward to his pelvis. His grip tugged at the tie again, momentarily choking off his partner’s retort.

“You seem keen on endeavouring to either bereft me of magic, or,” Gaster paused as he adjusted San’s grip on his tie with his natural hands, “soliciting your superior to have sex with you.” His mouth split into a wider grin, his cheeks dusting with a soft blush. “My, my. How forward.”

It didn’t stop him from giving a generous probe against the skeleton’s soul, however. The ethereal barrier shivered in his grip, opening to him as a series of increasingly ragged gasps came from Sans. A finger curled inside, passing the magic membrane and Gaster heard Sans almost squeak with the penetration.

“ff.. funny thing to say w-when you’re,  _hah,_ kn.. knuckle-deep in someone,” Sans rebutted, his body tensing when Gaster withdrew the finger long enough to inspect the construct giving him pleasure. Then back it went and thrust again, feeling the perverse wetness and heat of Sans’ soul clench around his digit with an accompanying high-pitched groan.

Gaster supposed he should have been a little more gentle. If the fluids slickening his grasp was anything to go by, Sans was dangerously close to orgasm already. He subjected the little skeleton to gentle rocking motions after that, teasing the cleft of his soul, pushing in with just enough firmness to stimulate. Sans was a particularly sensitive monster - Gaster figured it had something to do with not having a flesh-like membrane, but it went beyond that.

Touching his soul was always so satisfying. Sans would allow him, urge him on with frantic, needy groans. By the end of it, he’d be a sweating, exhausted mess, but it was something that always impressed Gaster.

Hands ready, the royal scientist nudged a second finger to join the first. He felt Sans tense with the addition, sounding like a blathering fool, he was so lost to the sensations that continued to bombard him. Gaster plucked at the underside of his ribs with another, just to be sure.

He could tell Sans was more into it than previously. There was always that telltale shiver and the way his hips jerked despite barely being bothered there. The skeleton was trying with all of his might to keep his lewd noises to a minimum, but even the squelching of his soul still made loud noise in the din of the surrounding darkness.

“I must tell you, I’m horribly inexperienced with dirty talk,” Gaster murmured, his voice like a purr as the skeleton beneath him began to writhe. Whatever he had said was likely to be humourous, as Sans gasped out a startled laugh, then squinted up at him, his magic collecting around his soul as the intrusions continued rocking.

“mm, i…” his assistant grunted. It was a valiant effort that he attempted something of a retort at all, Gaster supposed. The reaction was positively marvellous, however, and he needed to drink in the exquisite noises Sans was providing him attentively.

 

A shame that of all times for the power to go on, it was then. Gaster paused as the thrum of electronics flickered on with a signalling rush of beeps and Sans tensed in his grasp, obviously startled into silence, yet so delightfully flushed. The royal scientist surveyed him for a moment, then eyed the camera in the corner as he cautiously pulled up the skeleton’s trousers.

Still no recording light?

Well, then. He might as well take advantage of the light. Sans’ grip had begun to tremble on his lab coat, likely due to the fact that Gaster’s pause had landed them square in the middle of the skeleton’s soul. It pulsed around him and he could see how truly flushed and aroused his partner was, almost whimpering for him to resume.

“What would you do if I continued?” Gaster said, his voice hushed between them.

Sans’ eye sockets widened at that and he made an odd sound resembling a choked back moan. He tensed further, his soul fluttering as Gaster rubbed the interior walls of it, creating an aching pleasure that refused to ease. Instead of telling him to stop, however, Sans huffed as though attempting to disguise a sudden mewl of affirmation.

Gaster then saw him nod. If he was taken aback by such boldness, he did not show it. Instead, the slit of his bad eye glinting knowingly, he watched as his fingers explored every interior inch of the skeleton.

His ribs were sensitive, his magic thrummed with wakened arousal. Sans was trying so hard not to give away what was being done to him, despite losing control of his desperate noises. His eye sockets were lined with tears just threatening to fall and Gaster leaned over him to kiss them away. Sans’ rib cage was shuddering with every breath as the hands inside of it did their delightful evildoing.

Sans’ soul wept more as he drove inside a little faster, the movements being matched by Sans’ barely-audible panting. He was restraining himself  _just enough_ in order not to be picked up on the camera he likely supposed was recording.

But occasionally Gaster looked up to the camera, just in case. It was still offline. A passing thought figured that the power outage had fried the circuitry. Nonetheless, it was titillating, where he could fuck his lover’s soul and no one would find out about it.

He curled his fingers then as he thrusted and Sans’ essence clamped around him, protesting against the stimulation while the skeleton began to sound more at odds about restraining his appreciation. He huffed, inched himself up into Gaster’s free arm and hung onto him - giving Gaster that same feeling of being clung to like a koala.

But this time, it was out of desperation. It was well past what Sans could endure and his voice cracked on uneven groans and he fought for words. As he continued, Gaster wriggled the intrusion at the apex of the thrust, the result a shock of spasms that trickled down his palm.

Sans was moaning freely against him now, his body rocking with the movement as though to simulate the act. His hips twitched as the doctor pushed his free hand past the waistband to the ready magic below, thumbing the ready magic and Sans’ clit with a fluttering touch.

With Sans’ next encouraging reaction, Gaster matching the fingering in his soul with a defter rhythm below, using Sans’ desperate grunts and gasps as encouraging feedback. He cupped his fingers between the wet folds, using his thumb to rock against Sans’ clit at the same time.

Gaster cradled his face into the crook of Sans’ neck, his soul thundering as his lover’s hums and jerks of pleasure made tingles flood around his body like electricity. Sans’ legs attempted to cinch around his waist again, impeded by the doctor’s skill, driving the skeleton to grasping at his lab coat with renewed fervour.

“dings-” Sans gasped desperately, the single name uttered with such force that it felt raw in the best way. His hips rocked with the urgent tempo, the wet sounds oddly loud.

The doctor had lined up another finger to brush past the skeleton’s warm lips, filling and stretching Sans’ pussy as his digits created pressure along the conjured walls. Another gasp was huffed into his neck, ignoring himself while Gaster’s aim was to see just how far Sans would allow him to go.

The lights flickered again and he felt Sans relax slightly, as though seeing the act happening brought out his more timid side. Gaster chuckled and kissed the side of his skull, peppering light, adoring trails of kisses down Sans’ vertebrae until his boyfriend was whimpering.

The hand below changed angles and the doctor thrusted against him, his voice breaking softly as he whispered a little suggestion. “One more?”

Sans was less adamant on restraining his groan in retort, one of his legs pulling back to push Gaster closer to him. He nodded feverishly, his eye lights hazed through his stimulation while he anticipated a fourth finger to stretch him out, filling him completely and nudging against every spot of magic he could muster.

He gaped when the massaging to his soul gradually eased, but not for the reason that he was expecting. It was likely due to the reprieve, but Gaster decided to give him a little shock instead. The skeleton froze as a third finger nudged the apex of his soul, the quivering organ protesting with the proposed addition, and all of his being shuddered with the maddening heat.

“Is it too much?” the doctor enquired, pulling his head back to survey the skeleton’s face in the dim light. Sans’ mouth hung open, soft whistles of deep, urgent breaths passing between them as Gaster gently joined the third digit with the pair already inside of him.

“g.. go on,” the little skeleton urged stutteringly, his eye lights blown wide as more fluids ran down Gaster’s wrist. A soft ‘ah’ that sounded surprised more than anything escaped him, his rib cage jumping when he inhaled with the first withdrawal. When his soul was penetrated again, Gaster thought perhaps it was wise to ease him into it, despite how much he wanted to see his boyfriend a begging mess.

The affirmation was all he needed to continue. He chose to be caring, to leave soft kisses in his wake as Gaster moved the three fingers in and out of Sans’ soul. Every subtle shift and the tiniest sounds ignited his passion. The kisses were slow but deep, more sensual than the rough way Gaster had taken him before. He cupped the skeleton’s soul with one hand, using his thumbs to rub aching, caring magic against the sensitive organ.

Sans’ ragged panting had matched the gentler thrusts below, his hips moving in time, quickening just enough to tease. He whimpered - Gaster figured he was close enough, if the pain emanating from his shoulder was anything to go by. Sans had a deadly grip when it came to orgasms and restraining his noises.

Sans’ pussy clenched around his digits as he felt the throb pulse around them while he gave a sudden, choked cry of pleasure. The organ cradled in his rib cage fluttered around Gaster’s digits like a trapped bird, the buzz of magic sparking lights and colours as he rode out his orgasm.

Despite the wash of cyan peeking out from his assistant’s ribs, Gaster continued to give him pleasure, milking his desire for all that he was worth. It no longer mattered if the lights came on again, nor if the camera was actually recording. All that really mattered was the present moment, his boyfriend’s heightening cries of ecstasy and the desperate pulling of his clothes that he was being subjected to.

Of course it would be then that the power went on again. Giving in to a frustrated groan himself, Gaster buried his hand into Sans’ dripping sex once more, wriggling his reach deeper as Sans quashed the startled yelp that escaped him at suddenly being able to see the room again. Flushed and panting, Sans remained as still as he possibly could, as though waiting for an all-clear.

 

He didn’t receive one though. His ribs rose and fell in rhythm with Gaster’s excited huffs. He still felt the exploratory magic around and within his soul, stimulating him. It brought on the threat of a needy groan as Gaster’s hand brought him around through the afterglow and back again, this time focusing on the organ in his rib cage more than before.

He was starting to see pinpricks of light in his vision and he felt another intrusion, thicker than before. He recognised the slick being pushed against his pelvis and had to gasp in delight, pulling Gaster closer to him with jerky movements.

“w.. wondered when you’d g.. get impatient,” he moaned between them, his soul shuddering with excitement as he looked down to where his lover had hastily unbuckled his belt and drawn down his zipper. He took in the slick, dark appendage that seemed to ooze, lubricating in its own way as the tip was introduced to his opening.

He released a soft and nervous chuckle, the static of magic sending a confused wave of shivers all throughout his frame. He took a fistful of Gaster’s sweater as he came to a hilt inside of him, starting off small and stretching his magic out bit by bit. It throbbed and Sans focused on the sensation, very much aware that it wasn’t his pussy fluttering, by actually what Gaster’s body did naturally.

“Should I take that as a compliment?” the doctor enquired with a hushed chuckle of his own. He brought up one of Sans’ femurs and pushed further still, igniting sparks in Sans’ vision. “Is this alright?”

Sans couldn’t focus on anything else apart from the dual feeling spreading throughout his walls, seeping into everything that he had conjured. He mewled at the foreign sensation, but took solace in the fact nothing could actually harm him.

“nope. t.. terrible.”

Gaster clicked his tongue in mock admonishment and pulled back, only to thrust forward without pause. Sans’ passage tightened around him as he was filled with his girth and he spread his legs to invite him further, to drive deeper inside of him.

“Will you leave a bad review?” the doctor joked as he pulled one of Sans’ femurs up and over his shoulder.

Sans braced himself on the desk as Gaster’s rhythm continued and his eye sockets drifted closed. “w.. write it on the whiteboard in the meeting room-” he paused as a particularly wet squelching noise and his moan cut into the silence. “doctor gaster l.. loves tentacle sex,  _ah-”_

“Says the one currently indulging,” Gaster retorted with a subdued chortle, his efforts doubling as his member writhed inside of his lover. Sans covered his teeth and muffled another startled grunt, the phalanges of his other scratching at the desk when the probe nudged at just the right spot.

Sans was smiling and nodding, softer, needier mewls starting up again as his lover continued to pound into him. His legs twitched as the building pressure inside started up anew and he clawed at the desk, his body responding to the aching pleasure that was coursing through him.

Another whirr of electricity sounded along with a clang of lights coming on and he froze again, this time looking around him in a panic. His eye lights sought out the camera automatically and his soul fluttered in the scientist’s grasp.

There was that little red light.

Gulping quietly, he carefully shook his leg to get Gaster’s attention. “um-”

His partner looked up with his interruption, feeling a little soured by the constant surges of power. Gaster continued at a slower pace, still eliciting quiet huffs of effort from Sans in an attempt to keep quiet.

“Out with it,” he gently chided as he caressed the leg over his shoulder.

Sans looked back to him, his face flushed and his expression slightly mortified; “the, uh…”

He was interrupted by the sudden blaring voice over the intercom;  _“ATTENTION ALL EMPLOYEES. THIS CONCLUDES OUR EXERCISE FOR THIS MORNING. YOUR COOPERATION IS VERY MUCH APPRECIATED. SECTIONS A2, B4, G8, AND WD1 ARE NOW ONLINE. PLEASE CONTINUE YOUR WORK AND WE AGAIN THANK YOU FOR YOUR PATIENCE WHILE THE ROLLING BLACKOUTS HAVE RUN THEIR COURSE.”_

Both of them paused, heart and souls pounding in unison as it all immediately clicked into place. Gaster’s face flushed a combination of distraught and embarrassed shade of pink, while Sans covered his own face with both hands in an attempt to hide from the world.

 

Detaching more quickly than either of them could have possibly imagined, they helped each other right their clothes as footsteps were heard coming from down the hall. Gaster was just buttoning up Sans’ lab coat and attempting to detangle the faerie lights from around his collarbones when the door sounded, hissing and sliding to the side as a few of their late-rising colleagues entered the room.

Glances were stolen between the two partners and in silence, Sans and Gaster both vowed to run to the surveillance room as quickly as possible to destroy the evidence of their little tryst.

**Author's Note:**

> This is my Secret Santa gift for RaritysDiamonds!! It's my first Sanster work and I hope I nailed down things and it's fluffy and hot enough for you~~ HAPPY GYFTMAS HAVE SOME SMUT XD


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